fix: free ctx_copy in ggml_opt_free to plug per-training-session leak (#21592)

* fix: free ctx_copy in ggml_opt_free to plug per-training-session leak

ggml_opt_alloc populates opt_ctx->ctx_copy via a free+init pair every
time the allocated graph shape changes. The last ctx_copy from the
final ggml_opt_alloc call survives until ggml_opt_free is invoked,
but ggml_opt_free was only freeing ctx_static and ctx_cpu, never
ctx_copy. Each opt_ctx lifetime therefore leaks the final per-batch
context — ~900 KB for a typical GNN training session in
sindarin-pkg-tensor, surfaced via AddressSanitizer.

ctx_copy is nullptr-initialized and ggml_free() handles NULL safely,
so the new release is guard-free.

* Update ggml/src/ggml-opt.cpp

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

---------

Co-authored-by: realorko <realorko@nowhere.com>
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

ggml/src/ggml-opt.cpp

@@ -589,6 +589,7 @@ void ggml_opt_free(ggml_opt_context_t opt_ctx) {
     ggml_backend_buffer_free(opt_ctx->buf_cpu);
     ggml_free(opt_ctx->ctx_static);
     ggml_free(opt_ctx->ctx_cpu);
+    ggml_free(opt_ctx->ctx_copy);
     delete opt_ctx;
 }